Satellite antenna manufacturers are investigating high-frequency broadband satellite services for maritime vessels. In particular, spectrum in Ku Band and Ka Band is substantially broad and predominantly unused so as to provide an opportunity for economic broadband service.
High-frequency satellite transmissions typically increase the directivity of the satellite antenna. In this regard, the high-frequency transmissions typically can be received by the antenna only when the antenna is accurately pointed at the satellite. It is understood that the high degree of pointing accuracy increases the difficulty in both positioning the antenna and providing a long-term durable antenna. Namely, existing antenna, such as those on a vessel, receive vibrations that can sufficiently perturb the pointing direction or transmission toward the satellite.
With attention to FIGS. 1 and 2, one known antenna assembly 10 includes a spring suspension system 12 between an antenna 14 and a mast 16. As shown in FIG. 1, the spring suspension system 12 includes a series of springs 18 isolating vibration at the base 20 of the antenna 14 with the center of gravity 22 of the antenna 14 above the spring suspension system 12. Accordingly, the antenna 14 is somewhat movable on the mast 16 with the spring suspension system 12 affecting movement of the antenna 14. These springs 18 can be somewhat large with generally low coefficients of stiffness for minimizing low frequency vibrations. However, also in this regard, the antenna 14 deflects or sags in response to gravity and accelerations that are induced by ship motion. Examples of typical ship motion include roll, pitch, yaw, heave, surge, and sway. Referring now to FIG. 3, ship motion can cause antenna deflection and thus increase stabilization requirements for correcting the deflection or otherwise prevent the antenna from tracking a satellite under predetermined ship motions.
It will be appreciated that sufficiently large and soft springs 18 with resonances generally less than 4 Hz can typically attenuate the low-frequency vibration approximately between 4 Hz and 200 Hz. The vibrations typically are produced by rotating mechanisms of the vessel, such as the propeller, shaft, or engine assemblies. In particular, low-frequency vibrations can be transmitted from the propeller to the antenna assembly via structural components of the vessel. In addition, vibrations are also affected by sea conditions, vessel maneuvering, and vessel loading. However, the ship motion can cause the springs 18 to have substantially large deflections thereby requiring a significantly sized radome 24 and also producing a significant loss of tracking range. Pointing errors caused by the springs are greatest at low frequencies as deflection from vibration is proportional to acceleration divided by the vibration frequency squared. Accordingly, existing spring suspension systems 12 typically are tuned for isolating high-frequency vibration for providing durability rather than low-frequency vibration that provides pointing accuracy.
Referring back to FIGS. 1 and 2, the springs 18 are configured for rotating the antenna 14 and decreasing vibration stress. In particular, FIGS. 4 through 6 respectively show the wobble mode, the dangle mode, and the piston mode for the antenna 14. In these examples, the X-axis is positioned athwartship, with the Y-axis aligned along the longitudinal axis of the vessel and the Z-axis being vertical and aligned with gravity. The first two isolation modes about the X and Y-axes are similar in that each has a center of gravity substantially above the base 20 of the antenna 14. For this reason, vibration stress relief occurs through rotation for lateral and longitudinal vibration. Translational acceleration at the base 20 of the antenna 14 is not significantly affected by the lowest isolation mode.
Referring now to FIG. 7, there is shown a matrix of exemplary graphs for the transmittance of vibration to rotation of the antenna 14 at the base 20 and a top portion 26 of the antenna 14, respectively indicated by curve 28a and curve 28b. The X and Y inputs can produce rotation of the antenna 14, which is indicated by the difference between curve 28a and curve 28b. The relief of vibration stress by rotation creates small pointing changes.
Referring now to FIG. 8, it will be appreciated that the mass distribution of the antenna 14 (shown in FIG. 3) in conjunction with the movement of the antenna 14 typically cause additional rotational torque Rx, Ry, Rz about the respective axes. The rotational torque typically rotates the antenna 14 and thus adversely affects the pointing accuracy of the antenna 14. Rotation of the antenna 14 typically is prevented by pointing control mechanisms that apply corrective torque. Further, it is understood rotations that are induced by vibration occur at substantially high frequencies, namely from about 4 to 200 Hz. Accordingly, the spring suspension system 12 and pointing control mechanisms may require substantially high bandwidth control loops and significantly high torques for accurately pointing the antenna 14. This leads to larger motors, increased heat, higher cost, larger weight, increased power consumption and generally shortened life for antenna assembly drive components.
It would, therefore, be highly desirable to provide a vibration isolation system for an antenna assembly that enhances the pointing and tracking range and accuracy performance of the antenna assembly during use on a vehicle or vessel and minimizes the wear and control torque requirements on the same.